Not Applicable.
Not Applicable.
1. Field of Invention
The present invention relates to the control of the dimensions of bodies made from sinterable particulate materials. More specifically, this invention relates to the fabrication of commercial parts from sinterable materials in different sizes and from different materials with a minimal investment in tooling.
2. Description of Prior Art
With the exponential growth of technology and the globalization of markets, manufacturers of mass-produced products such as computers, cars, watches, and the vast array of today""s electronic consumer items, are challenged by an increasingly educated and discriminating consumer population expecting innovative, technically advanced and esthetically pleasing products incorporating the latest technological developments.
In order to remain competitive, manufacturers of such mass-produced items must be in a position to rapidly change the design of their products. This is not just to keep abreast of the fast changes in technology, but also to maintain a steady stream of new products with greater functionality and appealing esthetic appearance, e.g. different shapes, sizes, materials, external surface finish, colors, properties, etc., as demanded by the market.
Another market driven fad consists of fabricating a wide array of consumer items such as watch cases and bracelets, belt buckles, spectacle frames, and the like, from the exotic alloys used in the aerospace industry, e.g. titanium, superalloys, cobalt, rhodium, zirconia, tungsten carbide, etc., such materials carrying with them an image of high technology.
As a result, the economic life of many mass-produced consumer products is often very short, sometimesxe2x80x94as in the case of watches, computers or cellular phonesxe2x80x94only a mere six months. The new products, which must be produced in replacement of the obsolete ones, naturally imply substantial investments in new tooling. Also the raw materials may be costly or difficult to source. Machining may be laborious and difficult, particularly for extremely small or high precision parts, and generate excessive amounts of scrap which may be costly to recycle.
Understandably then, the prior art has attempted to make new products without incurring the huge costs of new tooling and without generating the scrap associated with conventional machining. One such net-shape manufacturing technique is the well-known powder metallurgy (P/M) process which forms parts by pressing metallic powders into simple shapes such as gears and pinions and then sinters these to a useful density. However P/M parts are generally pressed in a uniaxial direction, which limits the freedom of shape and may introduce density gradients in the pressed part, resulting in shape distortion upon sintering. The second drawback of the P/M process is the substantial amount of residual porosity after sintering, a condition that weakens the material properties of the parts and limits their use in commercial applications. For demanding applications P/M parts often require further densification, e.g. by isostatic pressing.
Another prior art method used to reduce machining costs is the powder injection molding process in which metal or ceramic powders are mixed with an organic binder and molded into green parts using the machinery and tooling of the plastics industry. Following extraction of the organic binder, the green parts are sintered as in the P/M process. This is usually accompanied by a substantial shrinkage, of the order of 20% linear, resulting in diminutives or miniatures of the corresponding green parts. However the shrinkage is not always isotropic resulting in poor dimensional control over the end products and the need for iterative empirical tooling adjustments.
When the new commercial products are not too different in size and configuration from the obsolete onesxe2x80x94like a new model of watch case or watch bracelet linkxe2x80x94the prior art at times attempts to reduce the cost of new tooling by modifying the existing tooling so that it can still be used for the new products. For instance, in the injection molding of plastic parts it is common to provide a mold with interchangeable cavity inserts, each insert being for a different product. However, the savings achieved by using new inserts in an existing mold base are often marginal, as it is precisely the fabrication of the molding inserts which constitutes the most laborious, time consuming and expensive part of building a molding tool.
The prior art has attempted to reduce the costs of mold inserts by making them via the powder injection molding route as disclosed, for example, in Amaya et al., U.S. Pat. No. 5,976,457. However, like any other powder injection molded parts, the green cavity inserts undergo shrinkage during sintering and, therefore, are likely to suffer from the same process weaknesses, namely part distortion and anisotropic shrinkage.
Yet another way the prior art has tried to reduce the cost of tooling is by making mold inserts from ceramic materials. One example is disclosed in Pluim, Jr. U.S. Pat. No. 4,704,079 who teaches a process whereby the mold inserts are formed by freeze casting particulate silicon metal and then reaction bonding the green inserts in nitrogen gas to generate silicon nitride. However, the dimensional accuracy of these inserts is generally poor so that extensive secondary machining is required in the green state first, using bonded carbide tools, and after sintering, using diamond tooling.
Still another way the prior art has attempted to reduce the cost of tooling is by extending the functionality of existing tooling into new products which are miniatures of those the existing tooling was originally designed for. This is of great interest when said miniatures are of such small dimensions that their fabrication by conventional means is extremely difficult, costly or even impossible. An example of such efforts at miniaturization is disclosed in Wiech, Jr., U.S. Pat. No. 5,234,655 where, through a series of iterative cycles, miniature parts are achieved. Each cycle consists of making a first green part in a first mold cavity. After sintering, during which the green part shrinks about 20% linearly, the product is used as a mold core in a second mold cavity and a second green part is produced which is a miniaturized mirror shape of the first green part. The second green part is then sintered in turn upon which, it too, shrinks about 20% linearly. The second sintered part is then used as a mold core in the cavity of a third mold to yield a third green part which is a miniature mirror shape of the second green part. Upon sintering of this third green part, a further 20% linear shrinkage takes place and a product, which constitutes a miniature of the first sintered product, is obtained. Wiech suggests that this cycle be repeated as many times as may be necessary to end up with a product that has the desired dimensions. The limitations of this procedure are obvious.
In conclusion, the prior art has not satisfactorily solved the problem of reducing the cost of new tooling or extending the functionality of existing tooling into new products.
Conceivably, this objective could be realized if products in a range of precisely predetermined sizes could be made using a single molding tool. Even more profit could be generated from existing tooling if one tool could also be used to make products of identical dimensions but from different materials. For example, a mold designed to produce plastic watch cases could be used to make stainless steel watch cases of different sizes, e.g. for gents, ladies and children. The same molding tool could also be used to make watch cases of identical dimensions but from different materials, e.g. stainless steel, tungsten carbide, zirconia, etc. This ideal situation would obviously only be possible if the different materials molded could be formulated with a precisely engineered and controlled shrinkage.
In accordance with the present invention the problems of the prior art are substantially overcome by providing a method to formulate an engineered thermoplastic mixture of sinterable particulate material and organic binder from which parts can be formed by prior art techniques such as molding, casting, machining and the like, the thus formed parts, upon extraction of the organic binder from them and sintering, displaying a uniform, isotropic, constant, and accurately predetermined shrinkage.
It is the primary object of this invention to provide a method to formulate a thermoplastic compound composed of at least one sinterable particulate material and at least one degradable thermoplastic ingredient, and displaying a predetermined, constant and isotropic shrinkage upon extraction of the thermoplastic constituent or constituents from said thermoplastic compound and sintering of the remaining particulate material or materials.
An additional object of this invention is to provide a method to control and optimize the shrinkage upon sintering of both moldable and non-moldable bodies produced from thermoplastic mixtures of sinterable particulates and organic binders.
It is another object of this invention to provide a method for reducing the investment in new tooling by making articles of identical or variable sizes and from different materials using existing tooling.
It is yet another object of this invention to provide an economical method for increasing the finctionality of existing tooling by extending its capabilities into products of different sizes and from different materials.
It is a further object of this invention to provide a method to produce miniatures of existing products without the need for additional or special tooling
It is yet a further object of this invention to provide a method that can be used to mass-produce small parts that cannot be produced easily, economically or at all, by conventional manufacturing techniques.